Spin-state variation in solid state and solution of mononuclear iron(II) 1,4,7-trimethyl-1,4,7-triazacyclonane complexes

The series of mononuclear iron(II) complexes with the tridentate macrocycle Me(3)tacn have been prepared (Me(3)tacn = 1,4,7-trimethyl-1,4,7-triazacyclonane). A purple, spin-crossover complex [Fe(Me(3)tacn)(MeCN)(3)](CF3SO3)(2) (1-OTf) forms in acetonitrile but readily loses MeCN ligands to form a colorless high-spin complex Fe(Me(3)tacn)(OTf)(2) (2). The BPh4- salt of 1 is stable to loss of MeCN and remains purple even under a vacuum. Methylene chloride solutions of Fe(OTf)(2) and Me(3)tacn afford crystals of [Fe(Me(3)tacn)(MeCN)(2)(OTf)](OTf) (3). Crystallization of 1-OTf in the presence of water affords a colorless high-spin complex, Fe(Me(3)tacn)(H2O)(CF3SO3)(2) (4), that exists as a pair of molecules bridged by hydrogen bonds between the coordinated water and the two bound triflate anions of the inversion-related partner. The crystallographic parameters are the following. 1-BPh4: C63H70B2Fe, monoclinic, P2(1)/c, a = 18.360(1) Angstrom, b = 11.761(1) Angstrom, c = 25.754(2) Angstrom, beta = 90.72(1)degrees, Z = 4. 3: C16H29Cl2F6FeN5O6S2, triclinic group P (1) over bar, a = 8.500(1) Angstrom, b = 11.421(2) Angstrom, c = 15.677(2) Angstrom, alpha = 92.23(1)degrees, beta = 94.79(1)degrees, gamma = 97.03(1)degrees, Z = 2. 4: C20H18F6FeN4O6S2, monoclinic, P2(1)/n, a = 11.253(3) Angstrom, b = 12.624(5) Angstrom, c = 14.683(5) Angstrom, beta = 94.02(2)degrees, Z = 4. Variable temperature visible spectra and H-1 NMR spectra of solutions of both 1-OTf and 1-BPh4 exhibit low-spin, high-spin crossover behavior, whereas 2, 3, and 4 remain high-spin in solution. The extensive role of coordinated triflate as a terminal and/or bridging ligand as well as a counteranion is demonstrated by variable temperature F-19 NMR spectra.